Breast cancer is one of the most common malignant tumors in women. More than one million new cases of breast cancer occur worldwide annually, and nearly 400 thousand people died from breast cancer every year. In recent years, the incidence of breast cancer showed a clear upward trend in the world. Despite in high and low endemic areas, the incidence of breast cancer increases by 5-20%. The common treatments for breast cancer include surgery, chemicotherapy and endocrine therapy and so on. Although these conventional treatments may prolong survival in patients to a large extent, their side effects are serious and their therapeutic effect is hard to be further improved. Targeted cancer therapy is a new treatment for cancer that has arisen in recent years, of which the representative is antitumor monoclonal antibody. HER2 (human epidermal growth factor receptor 2) is a transmembrane protein with tyrosine kinase activity, having a molecular weight of about 185 KD. Anti-HER2 humanized monoclonal antibody may specifically bind to HER2, and has antitumor mechanisms as follows: specifically binding to the extracellular domain of HER2 receptor to block the constitutive activation of HER2 homodimers and interfere the heterodimer formation of HER2 with other ErbB family members; mediating the endocytosis and the degradation in lysosomes of HER2 receptor; activating PTEN (phosphatase and tensin homology) and blocking PI3K (Phosphatidylinositol 3-kinase) signal channel; inhibiting tumor cell proliferation by regulation of cell cycle; promoting tumor cell apoptosis; inhibiting tumor angiogenesis; ADCC (antibody-dependent cell-mediated cytotoxicity) effect; inhibiting DNA repair; increasing the cytotoxicity of chemotherapeutic agents; reversing the resistance of tumor cells to the killing effects of host cell factors, and etc. (Pergram M, Ngo D, Application and potential limitations of animal models utilized in the development of trastuzumab (Herceptin®): A case study. Adv Drug Deliv Rev. 2006; 58:723-34 Anti-HER2 humanized monoclonal antibody (e.g. Trastuzumab, trade name: Herceptin) has been used in clinical trials to treat patients with HER2 overexpressing metastatic breast cancer as single drug, who had received but failed one or more chemotherapy regimens for their metastases. Response rates to single-agent trastuzumab range from 12 to 34% for metastatic breast cancer and significant improvements in survival rates are achieved in patients with early-stage HER2-overexpressing breast cancer in the adjuvant setting. However patients with a low expression of HER2 (i.e. HER2+ or HER2++ patients) are not eligible for a treatment with an anti-HER2 humanized monoclonal antibody such as Trastuzumab. Accordingly, there is a need for the treatment of patient having a cancer with a HER2 low expression.